Non-thiopurine methyltransferase related effects in 6-mercaptopurine therapy

ABSTRACT

The present invention provides methods for predicting tolerance associated with 6-mercaptopurine drug treatment of an immune-mediated gastrointestinal disorder such as inflammatory bowel disease. In particular, the present invention provides methods for predicting a patient&#39;s risk of an adverse drug reaction (or tolerance) to a 6-mercaptopurine drug by genotyping a patient at a polymorphic site in at least one gene selected from the group consisting of a xanthine dehydrogenase (XDH) gene, molybdenum cofactor sulfurase (MOCOS) gene, and aldehyde oxidase (AOX) gene. The present invention further provides methods for optimizing therapeutic efficacy in a patient receiving a 6-mercaptopurine drug by determining whether the patient should be given an alternative drug based on the presence or absence of a polymorphism in at least one of the XDH, MOCOS, and AOX genes.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No. 12/248,866, filed Oct. 9, 2008, which application claims priority to U.S. Provisional Patent Application No. 60/979,787, filed Oct. 12, 2007, the teachings of which are incorporated herein by reference in their entirety for all purposes.

FIELD OF THE INVENTION

The present invention provides a method for optimizing therapeutic efficacy and predicting tolerance of 6-mercaptopurine (6-MP) drug treatment, especially in an immune-mediated gastrointestinal disorder.

BACKGROUND OF THE INVENTION

Mercaptopurine (6-MP or 6-thiopurine) and azathioprine [6-(1-methyl-4-nitro-5-imidazolylthio) purine] are cytotoxic drugs that are effective in the treatment of ulcerative colitis and Crohn's disease (see, Present et al., Annals of Internal Medicine 111:641-649 (1989)). The prodrug azathioprine (AZA) is rapidly converted to 6-mercaptopurine through non-enzymatic, nucleophilic attack by sulfhydryl-containing compounds in the circulation. 6-MP and AZA, which are forms of the same drug and metabolic precursors of the active components, are acted upon by at least three competing enzymatic pathways. As shown in FIG. 1, several major enzyme pathways are involved. Xanthine oxidase (XO) converts 6-mercaptopurine to 6-thiouric acid. Hypoxanthine phosphoribosyl transferase (HPRT) converts 6-mercaptopurine to 6-thioinosine-5′-monophosphate, which is a precursor to 6-thioguanine nucleotides. Thiopurine methyltransferase (TPMT) catalyzes the S-methylation of 6-mercaptopurine to methylmercaptopurine (6-MMP). Thus, 6-mercaptopurine is enzymatically converted to various metabolites, including 6-thioguanine (6-TG) and 6-thioguanine nucleotides, which are the presumptive active metabolites mediating the effects of azathioprine/6-mercaptopurine drug therapy.

The interplay of the pathways described above is genetically determined and creates a highly individualized response to azathioprine/6-mercaptopurine drug therapy. The population frequency distribution of TPMT enzyme is trimodal, with the majority of individuals (89%) having high activity, 11% having intermediate activity, and about 1 in 300 (0.33%) having undetectable activity (see, Weinshilboum and Sladek, Amer. J. Human Genetics 32:651-662 (1980)). Such a trimodal relationship has been confirmed by direct measurements of TPMT enzyme activity by the Kroplin HPLC assay method (see, Kroplin et al., Eur. J. Clin. Pharmacol., 54 265-271 (1998)). In contrast to variation in TPMT activity, there is very little inter-individual variation in XO activity and only limited data on HPRT activity (see, Lennard, Eur. J. Clin. Pharm., 43:329-339 (1992)).

In certain populations, very high levels of methylated metabolites (e.g., 6-methyl-mercaptopurine (6-MMP)) are seen in red blood cells with normal thiopurine methyltransferase (TPMT) activity. This phenomenon has gone unexplained. There is contradictory evidence in the literature that high levels of methylated metabolites are associated with hepatotoxicity. Interestingly, when these patients are treated with a combination of allopurinol and azathioprine, methylated metabolites return to normal and thioguanine nucleotide levels can be pushed into the therapeutic range.

In view of the foregoing, there is a need in the art to understand the genetic interplay of the pathways described above as to create a highly individualized dose of a 6-mercaptopurine producing drug. The present invention satisfies this and other needs.

BRIEF SUMMARY OF THE INVENTION

The present invention provides methods for predicting a patient's risk of an adverse drug reaction (or tolerance) to a 6-mercaptopurine drug (e.g., AZA, 6-MP, or metabolites thereof) by genotyping a patient at a polymorphic site in at least one gene selected from the group consisting of a xanthine dehydrogenase (XDH) gene, molybdenum cofactor sulfurase (MOCOS) gene, and aldehyde oxidase (AOX) gene. The present invention further provides methods for optimizing therapeutic efficacy in a patient receiving a 6-mercaptopurine drug by determining whether the patient should be given an alternative drug based on the presence or absence of a polymorphism in at least one gene selected from the group consisting of a xanthine dehydrogenase (XDH) gene, molybdenum cofactor sulfurase (MOCOS) gene, and aldehyde oxidase (AOX) gene.

As such, the present invention provides a method for predicting clinical response or tolerance of a drug providing 6-mercaptopurine in an individual in need thereof, the method comprising:

-   -   (a) genotyping the individual at a polymorphic site in at least         one gene selected from the group consisting of a xanthine         dehydrogenase (XDH) gene, molybdenum cofactor sulfurase (MOCOS)         gene, aldehyde oxidase (AOX) gene, and a combination thereof;         and     -   (b) determining the presence or absence of a variant allele at         the polymorphic site, wherein the presence of the variant allele         at the polymorphic site is indicative of clinical response or         tolerance to the drug.

The methods described herein are useful in diseases or disorders such as an immune-mediated gastrointestinal disorder, an autoimmune disease, and graft versus host disease. The methods are especially useful in an immune-mediated gastrointestinal disorder such as inflammatory bowel disease, especially Crohn's disease.

In another embodiment, the present invention provides a method for predicting response to a drug providing 6-mercaptopurine in an individual in need thereof, the method comprising:

genotyping an aldehyde oxidase (AOX) gene of the individual for the presence or absence of a 3404 A>G (exon 30) variant allele, wherein the presence of the variant allele indicates that the individual should be given an alternative drug.

In yet another embodiment, the present invention provides a method for predicting tolerance of a drug providing 6-mercaptopurine in an individual in need thereof, the method comprising:

genotyping a xanthine dehydrogenase (XDH) gene of the individual for the presence or absence of a 837C>T (exon 10) variant allele, wherein the presence of the variant allele indicates that the individual is protected against side-effects to the drug.

Other objects, features, and advantages of the present invention will be apparent to one of skill in the art from the following detailed description and figure, which follows.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows the metabolism of azathioprine and 6-mercaptopurine. 6-mercaptopurine metabolic pathways are indicated by solid arrows; dashed arrows indicate putative products of dephosphorylation to nucleotides and further catabolism to nucleobases. HPRT, hypoxanthine phosphoribosyltransferase; TMPT, thiopurine methyltransferase; XO, xanthine oxidase; IMPD, inosine monophosphate dehydrogenase; GMPS, guanosine monophosphate synthetase.

DETAILED DESCRIPTION OF THE INVENTION I. Definitions

As used herein, the following terms have the meanings ascribed to them unless specified otherwise.

As used herein, the term “6-mercaptopurine drug” or “6-MP drug” includes any drug that can be metabolized to an active 6-mercaptopurine metabolite that has therapeutic efficacy such as 6-TG. Exemplary 6-mercaptopurine drugs as defined herein include 6-mercaptopurine (6-MP) and azathioprine (AZA). As illustrated in FIG. 1, both 6-MP and AZA can be metabolized to 6-mercaptopurine metabolites such as the exemplary 6-mercaptopurine metabolites shown, including 6-thioguanine (6-TG), 6-methyl-mercaptopurine (6-MMP), and 6-thiouric acid (see, Lennard, Eur. J. Clin. Pharmacol. 43:329 339 (1992)).

Other 6-MP drugs include, for example, 6-methylmercaptopurine riboside and 6-TG (see, Loo et al., Clin. Pharmacol. Ther. 9:180 194 (1968); O'Dwyer et al., J. Natl. Cancer Inst. 83:1235 1240 (1991); Erb et al., Cancer Chemother. Pharmacol. 42:266 272 (1998); Lancaster et al., Br. J. Haematol. 102:439 443 (1998); Ingle et al., Am. J. Clin. Oncol. 20:69 72 (1997); Evans and Relling, Leuk. Res. 18:811 814 (1994)). 6-TG is a particularly useful 6-MP drug in patients having high TPMT activity. Patients exhibiting high TPMT activity are expected to more easily convert 6-MP drugs such as 6-MP and AZA to 6-MMP (see, FIG. 1). As disclosed herein, high levels of 6-MMP are associated with hepatotoxicity. Therefore, patients with high TPMT activity can be more susceptible to toxic effects of 6-MP drug therapy. By administering 6-TG, which is an active 6-MP metabolite associated with therapeutic efficacy, the toxicity that can be associated with conversion of 6-MP to 6-MMP is bypassed.

As used herein, the term “6-thioguanine” or “6-TG” includes 6-thioguanine or analogues thereof, including molecules having the same base structure, for example, 6-thioguanine ribonucleoside, 6-thioguanine ribonucleotide mono-, di- and tri-phosphate, 6-thioguanine deoxyribonucleoside and 6-thioguanine deoxyribonucleotide mono, di, and triphosphate. The term “6-TG” also includes derivatives of 6-thioguanine, including chemical modifications of 6-TG, so long as the structure of the 6-TG base is preserved.

As used herein, the term “6-methyl-mercaptopurine” or “6-MMP” includes 6-methyl-mercaptopurine or analogues thereof, including analogues having the same base structure, for example, 6-methyl-mercaptopurine ribonucleoside, 6-methyl-mercaptopurine ribonucleotide mono-, di-, and tri-phosphate, 6-methyl-mercaptopurine deoxyribonucleoside, and 6-methyl-mercaptopurine deoxyribonucleotide mono-, di- and tri-phosphate. The term “6-MMP” also includes derivatives of 6-methyl-mercaptopurine, including chemical modifications of 6-MMP, so long as the structure of the 6-MMP base is preserved.

As used herein, the term “6-mercaptopurine metabolite” includes a product derived from 6-mercaptopurine in a biological system. Exemplary 6-mercaptopurine metabolites are shown in FIG. 1 and include 6-thioguanine (6-TG), 6-methyl-mercaptopurine (6-MMP) and 6-thiouric acid and analogues thereof. For example, 6-MP metabolites include 6-TG bases such as 6-TG, 6-thioguanosine mono-, di- and tri-phosphate; 6-MMP bases such as 6-methyl-mercaptopurine and 6-methyl-thioinosine monophosphate; 6-thioxanthosine (6-TX) bases such as 6-thioxanthosine mono-phosphate; 6-thioruric acid (6-TUA); and 6-MP bases such as 6-mercaptopurine and 6-thioinosine monophosphate. The immunosuppressive properties of 6-MP are believed to be mediated via the intracellular transformation of 6-MP to its active metabolites such as 6-TG and 6-MMP nucleotides. Furthermore, 6-MP metabolites such as 6-TG and 6-MMP were found to correlate with therapeutic efficacy and toxicity associated with 6-MP drug treatment of IBD patients.

The term “anti-inflammatory agent” includes any substance capable of preventing or reducing inflammation. Suitable anti-inflammatory agents include, without limitation, corticosteroids such as prednisolone, methylprednisolone aceponate, mometasone furoate, hydrocortisone, clobetasol propionate, betamethasone, betamethasone valerate, betamethasone dipropionate, dexamethasone, dexamethasone acetate, fluticasone propionate, clobetasone butyrate, beclomethasone dipropionate, and loteprednol etabonate; non-steroidal anti-inflammatory agents such as diclofenac, diflunisal, etodolac, fenbufen, fenoprofen, flurbiprofen, ibuprofen, indomethacin, ketoprofen, ketorolac, meclofenamate, mefenamic acid, meloxicam, nabumetone, naproxen, nimesulide, oxaprozin, piroxicam, salsalate, sulindac, tolmetin, celecoxib, rofecoxib, and 4-biphenylylacetic acid; antibodies such as infliximab; 5-aminosalicylates such as mesalamine, sulphasalazine, balsalazide, and olsalazine; antibiotics such as clindamycin, erythromycin, tetracycline, minocycline, doxycycline, penicillin, ampicillin, carbenicillin, methicillin, cephalosporins, vancomycin, bacitracin, streptomycin, gentamycin, chloramphenicol, fusidic acid, ciprofloxin and other quinolones, sulfonamides, trimethoprim, dapsone, isoniazid, teicoplanin, avoparcin, synercid, virginiamycin, cefotaxime, ceftriaxone, piperacillin, ticarcillin, cefepime, cefpirome, rifampicin, pyrazinamide, ciprofloxacin, levofloxacin, enrofloxacin, amikacin, netilmycin, imipenem, meropenem, and inezolid; pharmaceutically acceptable salts thereof; derivatives thereof; prodrugs thereof; and combinations thereof.

The term “immunosuppressive agent” includes any substance capable of producing an immunosuppressive effect, e.g., the prevention or diminution of the immune response, as by irradiation or by administration of drugs such as anti-metabolites, anti-lymphocyte sera, antibodies, etc. Suitable immunosuppressive agents include, without limitation, azathioprine and metabolites thereof such as those described above; anti-metabolites such as methotrexate; immunosuppressive antibodies such as anti-lymphocyte globulin antibodies, anti-thymocyte globulin antibodies, anti-CD3 antibodies, anti-CD4 antibodies, and antibody-toxin conjugates; mizoribine monophosphate; cyclosporine; scoparone; FK-506 (tacrolimus); FK-778; rapamycin (sirolimus); glatiramer acetate; mycopehnolate; pharmaceutically acceptable salts thereof; derivatives thereof; prodrugs thereof; and combinations thereof.

The term “gene” includes the segment of DNA involved in producing a polypeptide chain; it includes regions preceding and following the coding region, such as the promoter and 3′-untranslated region, respectively, as well as intervening sequences (introns) between individual coding segments (exons).

The term “nucleic acid” or “polynucleotide” includes deoxyribonucleotides or ribonucleotides and polymers thereof in either single- or double-stranded form including, for example, genomic DNA, cDNA and mRNA. This term encompasses nucleic acid molecules of both natural and synthetic origin as well as molecules of linear, circular, or branched configuration representing either the sense or antisense strand, or both, of a native nucleic acid molecule. It is understood that such nucleic acids can be unpurified, purified, or attached, for example, to a synthetic material such as a bead or column matrix. The term also encompasses nucleic acids containing known analogues of natural nucleotides that have similar binding properties as the reference nucleic acid and are metabolized in a manner similar to naturally occurring nucleotides. Unless otherwise indicated, a particular nucleic acid sequence also implicitly encompasses conservatively modified variants thereof (e.g., degenerate codon substitutions), polymorphisms, alleles, orthologs, SNPs, and complementary sequences as well as the sequence explicitly indicated. The term nucleic acid is used interchangeably with gene, cDNA, and mRNA encoded by a gene.

The term “polymorphism” includes the occurrence of two or more genetically determined alternative sequences or alleles in a population. A “polymorphic site” includes the locus at which divergence occurs. Preferred polymorphic sites have at least two alleles, each occurring at a particular frequency in a population. A polymorphic locus may be as small as one base pair (single nucleotide polymorphism, or SNP). Polymorphic markers include restriction fragment length polymorphisms, variable number of tandem repeats (VNTR's), hypervariable regions, minisatellites, dinucleotide repeats, trinucleotide repeats, tetranucleotide repeats, simple sequence repeats, and insertion elements such as Alu. The first identified allele is arbitrarily designated as the reference allele, and other alleles are designated as alternative alleles, “variant alleles,” or “variances.” The alleles occurring most frequently in a selected population is sometimes referred to as the “wild-type” allele. Diploid organisms may be homozygous or heterozygous for the variant alleles. The variant allele may or may not produce an observable physical or biochemical characteristic (“phenotype”) in an individual carrying the variant allele. For example, a variant allele may alter the enzymatic activity of a protein encoded by a gene of interest.

A “single nucleotide polymorphism” or “SNP” occurs at a polymorphic site occupied by a single nucleotide, which is the site of variation between allelic sequences. The site is usually preceded by and followed by highly conserved sequences of the allele (e.g., sequences that vary in less than 1/100 or 1/1000 members of the populations). A SNP usually arises due to substitution of one nucleotide for another at the polymorphic site. A transition is the replacement of one purine by another purine or one pyrimidine by another pyrimidine. A transversion is the replacement of a purine by a pyrimidine or vice versa. Single nucleotide polymorphisms can also arise from a deletion of a nucleotide or an insertion of a nucleotide relative to a reference allele.

The term “genotype” includes the genetic composition of an organism, including, for example, whether a diploid organism is heterozygous or homozygous for one or more variant alleles of interest.

The term “sample” includes any biological specimen obtained from a subject that contains nucleic acid. Suitable samples for use in the present invention include, without limitation, whole blood, plasma, serum, red blood cells, saliva, urine, stool (i.e., feces), tears, any other bodily fluid, tissue samples (e.g., biopsy), and cellular extracts thereof (e.g., red blood cellular extract).

The term “tolerance” includes the capacity of the body to endure a drug without an adverse drug reaction. In certain instances, the terms “adverse drug reaction” and “side-effect” include an undesirable secondary effect of a drug or therapy. Typical adverse drug reactions include, without limitation, bone marrow suppression, flu-like symptoms, rash, pancreatitis, nausea and vomiting, hepatotoxicity, neutropenia, and combinations thereof. In certain instances, “tolerance” means non-responsive to the therapy.

As used herein, the term “administering” includes oral administration, administration as a suppository, topical contact, intravenous, intraperitoneal, intramuscular, intralesional, intrathecal, intranasal or subcutaneous administration, or the implantation of a slow-release device, e.g., a mini-osmotic pump, to a subject. Administration is by any route, including parenteral and transmucosal (e.g., buccal, sublingual, palatal, gingival, nasal, vaginal, rectal, or transdermal). Parenteral administration includes, e.g., intravenous, intramuscular, intra-arteriole, intradermal, subcutaneous, intraperitoneal, intraventricular, and intracranial. Other modes of delivery include, but are not limited to, the use of liposomal formulations, intravenous infusion, transdermal patches, etc. By “co-administer” it is meant that a thiopurine drug such as AZA or 6-MP is administered at the same time, just prior to, or just after the administration of a second drug (e.g., anti-inflammatory agent, immunosuppressive agent, etc.).

II. General

Oral azathioprine is rapidly converted to 6-mercaptopurine (6-MP) by a nonenzymatic process. Initial 6-MP transformations occur along competing catabolic (XO, xanthine oxidase; TPMT) and anabolic (HPRT, hypoxanthine phosphoribosyltransferase) enzymatic pathways. Once formed by HPRT, 6-TIMP may be transformed into 6-TGN by the rate-limiting enzyme inosine monophosphate dehydrogenase (IMPDH) or methylated into 6-MMP (see, Dubinsky et al., Gastroenterology 118:705-713 (2000)). Other non-TGN mechanisms may also be at work.

Xanthine oxidase/dehydrogenase and aldehyde oxidase provide additional pathways for 6MP/AZA breakdown. Azathioprine is oxidized to 8-hydroxazathioprine by aldehyde oxidase. Xanthine oxidase (XO) converts 6-MP (and 6-TG following guanase conversion to thioxanthine) to thiouric acid (FIG. 1) in human liver and gut (and to a lesser extent in the kidney). Allopurinol inhibits xanthine oxidase, thus theoretically increasing the conversion efficiency of 6-MP to 6-TGN. Bone marrow toxicity arising from co-administration of allopurinol and 6-MP/AZA is well documented and this apparent increased efficacy has even been used as a basis for improving azathioprine response. Furthermore, raised erythrocyte 6-TGN has been demonstrated in the patients receiving allopurinol. The recommended rule of thumb is to reduce 6-MP/AZA dosage to a third or less of normal for a patient also receiving allopurinol.

III. Embodiments

The present invention provides methods for predicting a patient's risk of an adverse drug reaction or tolerance to a 6-mercaptopurine drug (e.g., AZA, 6-MP, or metabolites thereof) by genotyping a patient at a polymorphic site in at least one gene selected from the group consisting of a xanthine dehydrogenase (XDH) gene, molybdenum cofactor sulfurase (MOCOS) gene, aldehyde oxidase (AOX) gene, and a combination thereof. The present invention further provides methods for optimizing therapeutic efficacy in a patient receiving a 6-mercaptopurine drug by determining whether the patient should be given an alternative drug based on the presence or absence of a polymorphism in the xanthine dehydrogenase (XDH) gene, molybdenum cofactor sulfurase (MOCOS) gene, aldehyde oxidase (AOX), and a combination thereof.

The present methods are useful for diseases or disorders such as an immune-mediated gastrointestinal disorder, an autoimmune disease, and graft versus host disease. The methods are especially useful for an immune-mediated gastrointestinal disorder such as inflammatory bowel disease (IBD), e.g., Crohn's disease or ulcerative colitis.

In certain aspects, the methods of the present invention include at least two of the foregoing genes being genotyped. In certain other aspects, at least three of the genes are genotyped in a panel of genes. In certain other aspects, the method further includes genotyping TPMT.

In certain embodiments, the absence of the variant allele is indicative of decreased tolerance to the drug. Suitable 6-mercaptopurine drugs include, for example, 6-mercaptopurine, azathioprine, 6-thioguanine, and 6-methyl-mercaptopurine riboside. Preferably, the administered drug is 6-mercaptopurine or azathioprine.

In certain aspects, the method further includes minimizing a toxicity associated with the drug such as hepatic toxicity, hematological toxicity, and gastrointestinal toxicity.

A. Xanthine Dehydrogenase (XDH)

Xanthine oxidoreductase, which is a molybdenum hydroxylase, exists in two interconvertible forms, xanthine oxidase (EC 1.17.3.2) and xanthine dehydrogenase (EC 1.17.1.4). The conventional accepted role of xanthine oxidoreductase is purine catabolism, wherein it catalyzes the oxidation of hypoxanthine to xanthine and then to uric acid. Although the enzyme exists in two interconvertible forms, the same gene encodes the two enzymes. As used herein, the xanthine dehydrogenase (XDH) gene encodes both xanthine oxidase and xanthine dehydrogenase.

The human xanthine dehydrogenase (XDH) mRNA sequence is available under Genbank Accession No. NM_(—)000379 (SEQ ID NO:1), and the human XDH coding sequence (CDS) is set forth in SEQ ID NO:2. The human XDH genomic sequence is available under Genbank Accession Nos. NC_(—)000002 [REGION: complement (31410692 . . . 31491115)] and NT_(—)022184 [REGION: complement (10373121 . . . 10453544)].

With respect to the xanthine dehydrogenase (XDH) gene, a polymorphic site such as a variant allele selected from the group consisting of 2211C>T (exon 21) (SEQ ID NO:3), 3030T>C (exon 27) (SEQ ID NO:4), 837C>T (exon 10) (SEQ ID NO:5), 3717G>A (exon 34) (SEQ ID NO:6), 2107A>G (exon 20) (SEQ ID NO:7), 1936A>G (exon 18) (SEQ ID NO:8), and a combination thereof is useful in the present methods. The number (e.g., “2211”) in front of each nucleotide substitution (e.g., “C>T”) corresponds to the position of that particular nucleotide substitution in the human XDH coding sequence (SEQ ID NO:2). The exon designation (e.g., “exon 21”) refers to the specific exon of the human XDH genomic sequence in which the nucleotide substitution is located. For example, “837C>T (exon 10)” corresponds to a C to T nucleotide substitution at position 837 of SEQ ID NO:2; this polymorphism is located in exon 10 of the human XDH genomic sequence. The 837C>T (exon 10) variant allele is especially useful in the present methods.

As described in Example 1, the presence of the 837C>T (exon 10) variant allele protects against side-effects of drugs that produce 6-mercaptopurine. In individuals having this polymorphism, normal drug doses are administered without adverse side-effects.

B. Molybdenum Cofactor Sulfurase (MOCOS)

Polymorphisms in the human molybdenum cofactor sulfurase (MOCOS) gene are also useful in the present methods. The human MOCOS mRNA sequence is available under Genbank Accession No. NM_(—)017947 (SEQ ID NO:9), and the human MOCOS coding sequence (CDS) is set forth in SEQ ID NO:10. The human MOCOS genomic sequence is available under Genbank Accession Nos. NC_(—)000018 [REGION: 32021478 . . . 32102683] and NT_(—)010966 [REGION: 15256582 . . . 15337787].

In certain instances, the polymorphic site is a variant allele in the molybdenum cofactor sulfurase (MOCOS) gene selected from the group consisting of 2107C>A (exon 11) (SEQ ID NO:11), 509C>T (exon 4) (SEQ ID NO:12), 1072G>A (exon 6) (SEQ ID NO:13), 2600T>C (exon 15) (SEQ ID NO:14), 359G>A (exon 4) (SEQ ID NO:15), and a combination thereof. The number (e.g., “2107”) in front of each nucleotide substitution (e.g., “C>A”) corresponds to the position of that particular nucleotide substitution in the human MOCOS coding sequence (SEQ ID NO:10). The exon designation (e.g., “exon 11”) refers to the specific exon of the human MOCOS genomic sequence in which the nucleotide substitution is located. For example, “2107C>A (exon 11)” corresponds to an A to C nucleotide substitution at position 2107 of SEQ ID NO:10; this polymorphism is located in exon 11 of the human MOCOS genomic sequence. The 2107C>A (exon 11) variant allele is especially useful in the present methods.

In certain individuals, 509C>T, 1072G>A, and 359G>A are very strongly linked and almost always occur together. Two of these SNPs are situated close together in exon 4 and the third (1072G>A) is in exon 6.

As described in Example 1, the presence of the 2107C>A (exon 11) variant allele protects against side-effects of drugs that produce 6-mercaptopurine. In individuals having this polymorphism, normal drug doses are administered without adverse side-effects.

C. Aldehyde Oxidase (AOX) Gene

Aldehyde oxidase (EC 1.2.3.1) is another molybdenum hydroxylase. This cytosolic flavoenzyme generally catalyzes nucleophilic oxidation of N-heterocycles. The complex flavoprotein comprises two identical subunits of molecular weight of 145,000. Each subunit contains one molybdenum, one FAD, and two nonidentical, iron sulfur redox centers as an electron reservoir.

The human aldehyde oxidase (AOX) coding sequence is available under Genbank Accession No. NM_(—)001159 (SEQ ID NO:16), and the human AOX coding sequence (CDS) is set forth in SEQ ID NO:17. The human AOX genomic sequence is available under Genbank Accession Nos. NC_(—)000002 [REGION: 201158976 . . . 201244463] and NT_(—)005403 [REGION: 51660148 . . . 51745635].

With respect to the AOX gene, a polymorphic site such as a 3404A>G (exon 30) (SEQ ID NO:18) variant allele is useful in the present methods. The number (e.g., “3404”) in front of the nucleotide substitution (e.g., “A>G”) corresponds to the position of that particular nucleotide substitution in the human AOX coding sequence (SEQ ID NO:17). The exon designation (e.g., “exon 30”) refers to the specific exon of the human AOX genomic sequence in which the nucleotide substitution is located. For example, “3404A>G (exon 30)” corresponds to an A to G nucleotide substitution at position 3404 of SEQ ID NO:17; this polymorphism is located in exon 30 of the human AOX genomic sequence.

As described in Example 1, the presence of the 3404A>G (exon 30) variant allele indicates that the individual should be given an alternative drug as a non-responder. In certain aspects, the present invention provides a method for predicting response to a drug providing 6-mercaptopurine in an individual in need thereof, comprising genotyping the aldehyde oxidase gene for the presence of the 3404 A>G (exon 30) variant allele, wherein the presence of the variant allele indicates that the individual should be given an alternative drug.

In certain other optional embodiments, TPMT genotyping is also conducted. TPMT genotyping is useful for predicting the effectiveness of 6-MP therapy in an IBD patient. Heterozygote patients are expected to have lower TPMT activity and should therefore be monitored for high levels of 6-TG for possible toxic levels associated with leukopenia or bone marrow suppression. Homozygous patients deficient in TPMT activity can be treated with lower doses of a 6-MP drug provided that patients are closely monitored for toxicity such as leukopenia. Therefore, TPMT genotyping can be used to predict patient responsiveness to and potential toxicities associated with 6-MP drug therapy. Furthermore, TPMT genotyping can be combined with other methods of the invention to both determine TPMT genotype and to monitor 6-MP metabolites. TPMT genotyping can be particularly valuable when determining a starting dose of 6-MP drug therapy, but can also be useful when adjusting 6-MP drug doses after therapy has begun.

IV. Methods of Genotyping

A variety of means can be used to genotype a subject at a polymorphic site in at least one gene selected from the group consisting of a xanthine dehydrogenase (XDH) gene, molybdenum cofactor sulfurase (MOCOS) gene, and aldehyde oxidase (AOX) gene in the methods of the present invention in order to determine whether a sample (e.g., a nucleic acid sample) contains at least one variant allele. For example, enzymatic amplification of nucleic acid from a subject can be conveniently used to obtain nucleic acid for subsequent analysis. The presence or absence of a variant allele in at least one gene selected from the group consisting of a xanthine dehydrogenase (XDH) gene, molybdenum cofactor sulfurase (MOCOS) gene, and aldehyde oxidase (AOX) gene can also be determined directly from the subject's nucleic acid without enzymatic amplification.

Genotyping of nucleic acid from a subject, whether amplified or not, can be performed using any of various techniques. Useful techniques include, without limitation, polymerase chain reaction (PCR) based analysis, sequence analysis, and electrophoretic analysis, which can be used alone or in combination. As used herein, the term “nucleic acid” means a polynucleotide such as a single- or double-stranded DNA or RNA molecule including, for example, genomic DNA, cDNA and mRNA. This term encompasses nucleic acid molecules of both natural and synthetic origin as well as molecules of linear, circular, or branched configuration representing either the sense or antisense strand, or both, of a native nucleic acid molecule. It is understood that such nucleic acids can be unpurified, purified, or attached, for example, to a synthetic material such as a bead or column matrix.

Material containing nucleic acid is routinely obtained from subjects. Such material is any biological matter from which nucleic acid can be prepared. As non-limiting examples, material can be whole blood, plasma, saliva, cheek swab, or other bodily fluid or tissue that contains nucleic acid. In one embodiment, a method of the present invention is practiced with whole blood, which can be obtained readily by non-invasive means and used to prepare genomic DNA. In another embodiment, genotyping involves amplification of a subject's nucleic acid using the polymerase chain reaction (PCR). Use of PCR for the amplification of nucleic acids is well known in the art (see, e.g., Mullis et al. (Eds.), The Polymerase Chain Reaction, Birkhäuser, Boston, (1994)). In yet another embodiment, PCR amplification is performed using one or more fluorescently labeled primers. In a further embodiment, PCR amplification is performed using one or more labeled or unlabeled primers that contain a DNA minor grove binder.

Any of a variety of different primers can be used to amplify a subject's nucleic acid by PCR. As understood by one skilled in the art, additional primers for PCR analysis can be designed based on the sequence flanking the polymorphic site(s) of interest. As a non-limiting example, a sequence primer can contain from about 15 to about 30 nucleotides of a sequence upstream or downstream of the polymorphic site of interest. Such primers generally are designed to have sufficient guanine and cytosine content to attain a high melting temperature which allows for a stable annealing step in the amplification reaction. Several computer programs, such as Primer Select, are available to aid in the design of PCR primers.

A Taqman® allelic discrimination assay available from Applied Biosystems can be useful for genotyping an individual at a polymorphic site and thereby determining the presence or absence of a variant allele. In a Taqman® allelic discrimination assay, a specific fluorescent dye-labeled probe for each allele is constructed. The probes contain different fluorescent reporter dyes such as FAM and VIC to differentiate amplification of each allele. In addition, each probe has a quencher dye at one end which quenches fluorescence by fluorescence resonance energy transfer. During PCR, each probe anneals specifically to complementary sequences in the nucleic acid from the subject. The 5′ nuclease activity of Taq polymerase is used to cleave only probe that hybridizes to the allele. Cleavage separates the reporter dye from the quencher dye, resulting in increased fluorescence by the reporter dye. Thus, the fluorescence signal generated by PCR amplification indicates which alleles are present in the sample. Mismatches between a probe and allele reduce the efficiency of both probe hybridization and cleavage by Taq polymerase, resulting in little to no fluorescent signal. Those skilled in the art understand that improved specificity in allelic discrimination assays can be achieved by conjugating a DNA minor grove binder (MGB) group to a DNA probe as described, e.g., in Kutyavin et al., Nuc. Acids Research 28:655-661 (2000). Minor grove binders include, but are not limited to, compounds such as dihydrocyclopyrroloindole tripeptide (DPI3).

Sequence analysis can also be useful for genotyping a subject at a polymorphic site. A variant allele can be detected by sequence analysis using the appropriate primers, which are designed based on the sequence flanking the polymorphic site of interest, as is known by those skilled in the art. As a non-limiting example, a sequence primer can contain from about 15 to about 30 nucleotides of a sequence that corresponds to a sequence about 40 to about 400 base pairs upstream or downstream of the polymorphic site of interest. Such primers are generally designed to have sufficient guanine and cytosine content to attain a high melting temperature which allows for a stable annealing step in the sequencing reaction.

The term “sequence analysis” means any manual or automated process by which the order of nucleotides in a nucleic acid is determined. As an example, sequence analysis can be used to determine the nucleotide sequence of a sample of DNA. The term sequence analysis encompasses, without limitation, chemical and enzymatic methods such as dideoxy enzymatic methods including, for example, Maxam-Gilbert and Sanger sequencing as well as variations thereof. The term sequence analysis further encompasses, but is not limited to, capillary array DNA sequencing, which relies on capillary electrophoresis and laser-induced fluorescence detection and can be performed using instruments such as the MegaBACE 1000 or ABI 3700. As additional non-limiting examples, the term sequence analysis encompasses thermal cycle sequencing (see, Sears et al., Biotechniques 13:626-633 (1992)); solid-phase sequencing (see, Zimmerman et al., Methods Mol. Cell Biol. 3:39-42 (1992); and sequencing with mass spectrometry, such as matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (see, MALDI-TOF MS; Fu et al., Nature Biotech. 16:381-384 (1998)). The term sequence analysis further includes, but is not limited to, sequencing by hybridization (SBH), which relies on an array of all possible short oligonucleotides to identify a segment of sequence (see, Chee et al., Science 274:610-614 (1996); Drmanac et al., Science 260:1649-1652 (1993); and Drmanac et al., Nature Biotech. 16:54-58 (1998)). One skilled in the art understands that these and additional variations are encompassed by the term sequence analysis as defined herein.

Electrophoretic analysis also can be useful in genotyping a subject according to the methods of the present invention. “Electrophoretic analysis” as used herein in reference to one or more nucleic acids such as amplified fragments means a process whereby charged molecules are moved through a stationary medium under the influence of an electric field. Electrophoretic migration separates nucleic acids primarily on the basis of their charge, which is in proportion to their size, with smaller molecules migrating more quickly. The term electrophoretic analysis includes, without limitation, analysis using slab gel electrophoresis, such as agarose or polyacrylamide gel electrophoresis, or capillary electrophoresis. Capillary electrophoretic analysis generally occurs inside a small-diameter (50-100 m) quartz capillary in the presence of high (kilovolt-level) separating voltages with separation times of a few minutes. Using capillary electrophoretic analysis, nucleic acids are conveniently detected by UV absorption or fluorescent labeling, and single-base resolution can be obtained on fragments up to several hundred base pairs. Such methods of electrophoretic analysis, and variations thereof, are well known in the art, as described, for example, in Ausubel et al., Current Protocols in Molecular Biology Chapter 2 (Supplement 45) John Wiley & Sons, Inc. New York (1999).

Restriction fragment length polymorphism (RFLP) analysis can also be useful for genotyping a subject at a polymorphic site in the ITPA gene according to the methods of the present invention (see, Jarcho et al. in Dracopoli et al., Current Protocols in Human Genetics pages 2.7.1-2.7.5, John Wiley & Sons, New York; Innis et al., (Ed.), PCR Protocols, San Diego: Academic Press, Inc. (1990)). As used herein, “restriction fragment length polymorphism analysis” includes any method for distinguishing polymorphic alleles using a restriction enzyme, which is an endonuclease that catalyzes degradation of nucleic acid following recognition of a specific base sequence, generally a palindrome or inverted repeat. One skilled in the art understands that the use of RFLP analysis depends upon an enzyme that can differentiate a variant allele from a wild-type or other allele at a polymorphic site.

In addition, allele-specific oligonucleotide hybridization can be useful for genotyping a subject in the methods of the present invention. Allele-specific oligonucleotide hybridization is based on the use of a labeled oligonucleotide probe having a sequence perfectly complementary, for example, to the sequence encompassing the variant allele. Under appropriate conditions, the variant allele-specific probe hybridizes to a nucleic acid containing the variant allele but does not hybridize to the one or more other alleles, which have one or more nucleotide mismatches as compared to the probe. If desired, a second allele-specific oligonucleotide probe that matches an alternate (e.g., wild-type) allele can also be used. Similarly, the technique of allele-specific oligonucleotide amplification can be used to selectively amplify, for example, a variant allele by using an allele-specific oligonucleotide primer that is perfectly complementary to the nucleotide sequence of the variant allele but which has one or more mismatches as compared to other alleles (Mullis et al., supra). One skilled in the art understands that the one or more nucleotide mismatches that distinguish between the variant allele and other alleles are often located in the center of an allele-specific oligonucleotide primer to be used in the allele-specific oligonucleotide hybridization. In contrast, an allele-specific oligonucleotide primer to be used in PCR amplification generally contains the one or more nucleotide mismatches that distinguish between the variant and other alleles at the 3′ end of the primer.

A heteroduplex mobility assay (HMA) is another well-known assay that can be used for genotyping at a polymorphic site in the methods of the present invention. HMA is useful for detecting the presence of a variant allele since a DNA duplex carrying a mismatch has reduced mobility in a polyacrylamide gel compared to the mobility of a perfectly base-paired duplex (see, Delwart et al., Science, 262:1257-1261 (1993); White et al., Genomics, 12:301-306 (1992)).

The technique of single strand conformational polymorphism (SSCP) can also be useful for genotyping at a polymorphic site in the methods of the present invention (see, Hayashi, Methods Applic., 1:34-38 (1991)). This technique is used to detect variant alleles based on differences in the secondary structure of single-stranded DNA that produce an altered electrophoretic mobility upon non-denaturing gel electrophoresis. Variant alleles are detected by comparison of the electrophoretic pattern of the test fragment to corresponding standard fragments containing known alleles.

Denaturing gradient gel electrophoresis (DGGE) can be useful in the methods of the present invention. In DGGE, double-stranded DNA is electrophoresed in a gel containing an increasing concentration of denaturant; double-stranded fragments made up of mismatched alleles have segments that melt more rapidly, causing such fragments to migrate differently as compared to perfectly complementary sequences (see, Sheffield et al., “Identifying DNA Polymorphisms by Denaturing Gradient Gel Electrophoresis” in Innis et al., supra, 1990).

Other molecular methods useful for genotyping a subject at a polymorphic site are known in the art and useful in the methods of the present invention. Such well-known genotyping approaches include, without limitation, automated sequencing and RNAase mismatch techniques (see, Winter et al., Proc. Natl. Acad. Sci., 82:7575-7579 (1985)). Furthermore, one skilled in the art understands that, where the presence or absence of multiple variant alleles is to be determined, individual variant alleles can be detected by any combination of molecular methods. See, in general, Birren et al. (Eds.) Genome Analysis: A Laboratory Manual Volume 1 (Analyzing DNA) New York, Cold Spring Harbor Laboratory Press (1997). In addition, one skilled in the art understands that multiple variant alleles can be detected in individual reactions or in a single reaction (a “multiplex” assay).

In view of the above, one skilled in the art realizes that the methods of the present invention for predicting tolerance or optimizing therapeutic efficacy to a thiopurine drug by genotyping a subject in at least one gene selected from the group consisting of xanthine dehydrogenase (XDH) gene, molybdenum cofactor sulfurase (MOCOS) gene, and aldehyde oxidase (AOX) gene can be practiced using one or any combination of the well-known assays described above or other assays known in the art.

V. Diseases

The methods of the invention relate to treatment of an immune-mediated gastrointestinal disorder. As used herein, the term “immune-mediated gastrointestinal disorder” or “immune-mediated GI disorder” includes a non-infectious disease of the gastrointestinal tract or bowel that is mediated by the immune system or cells of the immune system. Immune-mediated gastrointestinal disorders include, for example, inflammatory bowel diseases (IBD) such as Crohn's disease and ulcerative colitis, lymphocytic colitis, microscopic colitis, collagenous colitis, autoimmune enteropathy, allergic gastrointestinal disease and eosinophilic gastrointestinal disease.

The methods of the invention are particularly useful for treating IBD, or subtypes thereof, which has been classified into the broad categories of Crohn's disease and ulcerative colitis. As used herein, “a subject having inflammatory bowel disease” is synonymous with the term “a subject diagnosed with having an inflammatory bowel disease,” and means a patient having Crohn's disease or ulcerative colitis. Crohn's disease (regional enteritis) is a disease of chronic inflammation that can involve any part of the gastrointestinal tract. Commonly, the distal portion of the small intestine (ileum) and cecum are affected. In other cases, the disease is confined to the small intestine, colon or anorectal region. Crohn's disease occasionally involves the duodenum and stomach, and more rarely the esophagus and oral cavity.

Ulcerative colitis (UC) is a disease of the large intestine characterized by chronic diarrhea with cramping abdominal pain, rectal bleeding, and loose discharges of blood, pus and mucus. The manifestations of ulcerative colitis vary widely. A pattern of exacerbations and remissions typifies the clinical course of most UC patients (70%), although continuous symptoms without remission are present in some patients with UC. Local and systemic complications of UC include arthritis, eye inflammation such as uveitis, skin ulcers and liver disease. In addition, ulcerative colitis and especially long-standing, extensive disease is associated with an increased risk of colon carcinoma.

In comparison with Crohn's disease, which is a patchy disease with frequent sparing of the rectum, ulcerative colitis is characterized by a continuous inflammation of the colon that usually is more severe distally than proximally. The inflammation in ulcerative colitis is superficial in that it is usually limited to the mucosal layer and is characterized by an acute inflammatory infiltrate with neutrophils and crypt abscesses. In contrast, Crohn's disease affects the entire thickness of the bowel wall with granulomas often, although not always, present. Disease that terminates at the ileocecal valve, or in the colon distal to it, is indicative of ulcerative colitis, while involvement of the terminal ileum, a cobblestone-like appearance, discrete ulcers or fistulas suggest Crohn's disease. 5′-monophosphate. Measuring 6-MMP can include 6-methylmercaptopurine and 6-methylthioinosine 5′-monophosphate, and can also include 6-methylthioinosine di- and tri-phosphate, as well as 6-methyl thioguanosine.

VI. Examples

The following examples are intended to illustrate but not limit the present invention.

Example 1

This example illustrates the use of the various SNPs of the present invention.

A. Patients

Patients were recruited at the point of starting azathioprine therapy. One hundred and sixty-eight (168) patients from this cohort were included in the study, selected on the availability of complete clinical data and adequate DNA. Complete remission was defined by symptom scores and absence of steroid use. Treatment failure was defined by recourse to surgery, alternative immunomodulator or biologic therapy. Side-effects were included in the analysis only if they were the cause of treatment withdrawal.

B. Laboratory Methods

1. DNA Extraction

DNA was extracted from whole blood collected in EDTA bottle using the QIAmp DNA Mini Kit 250 (Qiagen Ltd. Crawley, UK). Briefly, 200 μl of whole blood was lysed by addition of protease enzyme and buffer AL from the Qiagen kit. This mixture was vortexed and then incubated at 56° C. for 10 minutes to digest and denature blood proteins. 200 μl of 100% ethanol was then added, the mixture was vortexed and transferred to a QIAmp spin column within a 2 ml collection tube. These tubes were then centrifuged at 8000 rpm for 1 minute to adsorb the DNA onto the silica-gel membrane of the spin column. The column then underwent washing steps to remove contaminants that could interfere with the PCR reaction. The column was then transferred to a clean collection tube and the DNA eluted from the column membrane by addition of 200 μl of QIAgen buffer AE and a final centrifuge step of 8000 rpm for 1 minute. On average, this method produced 6 μg of total DNA in 20-30 kb lengths from 200 μl of whole blood. This DNA was mixed with 50 μl of tris-EDTA (×1 mixture) to inhibit DNAases and stored in a freezer at −20° C.

2. Real-Time PCR

SNPs were selected for analysis in the three target genes: xanthine dehydrogenase (XDH), molybdenum cofactor sulfurase, (MOCOS) and aldehyde oxidase (AOX). Only coding SNPs were selected and the choice was further narrowed according to reported gene frequencies in the Caucasian population and, where possible, by choosing SNPs that encoded a non-conservative change in an amino acid residue. Probes for the SNPs selected were obtained from Applied Biosystems (Warrington, UK). Details are shown in Table 1.

Patients were genotyped by real-time PCR using a Biorad Miniopticon (Bio-Rad, Hemel Hempstead, UK). 1.8 μl of DNA was mixed with Absolute QPCR Mix (Abgene, Epsom, UK) and SNP mix (Applied Biosystems, Warrington, UK) and diluted up to volume with DNA-free water, according to the manufacturers' instructions. PCR conditions were 15 minutes enzyme activation at 95° C., then 42 cycles of: denaturation (15 secs at 95° C.) and anneal/extension (1 min at 60° C.).

TABLE 1 SNP information. dbSNP cDNA base Amino acid Fre- rs number Gene Exon change substitution quency rs2295475 XDH 2p23.1a 21 2211C > T Ile737Ile 0.31 rs1884725 XDH 2p23.1a 27 3030T > C Phe1010Phe 0.23 rs4407290 XDH 2p23.1a 10  837C > T Val279Val 0.02 rs207440 XDH 2p23.1a 34 3717G > A Glu1239Glu 0.06 rs17011368 XDH 2p23.1a 20 2107A > G Ile703Val 0.05 rs17323225 XDH 2p23.1a 18 1936A > G Ile646Val 0.05 rs594445 MOCOS 11 2107C > A His703Asn 0.34 18q12.2a rs623053 MOCOS 4  509C > T Thr170Ile 0.03 18q12.2a rs678560 MOCOS 6 1072G > A Val358Met 0.03 18q12.2a rs1057251 MOCOS 15 2600T > C Val867Ala 0.10 18q12.2a rs3744900 MOCOS 4  359G > A Ser120Asn 0.03 18q12.2a n/a AOX 2q33.1e 30 3404A > G Asn1135Ser 0.16 C—cytosine, T—thymine, G—guanine, A—adenine. Ile—isoleucine, Phe—phenylalanine, Val—valine, Glu—glutamate, Asn—asparagine, His—histidine, Thr—threonine, Met—methionine, Ala—alanine. Frequencies are those quoted for the Caucasian population.

3. Statistics

Associations between side-effects and genotype were determined using contingency tables and Chi-squared and Fisher exact tests were applied. Effect sizes were measured using odds ratios and confidence intervals.

C. Results

The gene frequencies identified were similar to those reported in SNP databases. The details of these frequencies are displayed in Table 2.

TABLE 2 Gene frequencies in the present cohort compared with reported frequencies in SNP databases (http://www.ncbi.nlm.nih.gov/SNP). SNP Expected frequency Documented frequency XDH 2211C > T 0.31 0.25 XDH 3030T > C 0.23 0.23 XDH 837C > T 0.02 0.04 XDH 3717G > A 0.06 0.06 XDH 2107A > G 0.05 0.08 XDH 1936A > G 0.05 0.08 MOCOS 2107C > A 0.34 0.29 MOCOS 509C > T 0.03 0.06 MOCOS 1072G > A 0.03 0.06 MOCOS 2600T > C 0.10 0.10 MOCOS 359G > A 0.03 0.05 AOX 3404A > G 0.16 0.12

SNPs MOCOS 509C>T, 1072G>A, and 359G>A were very strongly linked and almost always occurred together. Two of these SNPs were situated close together in exon 4 and the third (MOCOS 1072G>A) was at quite a distance in exon 6. In analysis of functional relevance, these SNPs have therefore been analyzed together.

SNPs were analyzed for association with side-effects or non-response. The SNP XDH 837C>T was found to protect against side-effects to azathioprine (p=0.046). A trend towards protection from side-effects was seen in a few other SNPs in both XDH and MOCOS. Removing those side-effects which are already accounted for by a TPMT polymorphism from the analysis strengthened the association between SNP MOCOS 2107C>A and protection against side-effects. The strongest association detected was between the presence of SNP AOX 3404A>G and a lack of response to azathioprine (p=0.006).

In total, there were 35 patients deemed to have no response to azathioprine and 7 who were partial responders. TGN levels were available for 34 of these patients. These levels would suggest that 11 of this group were poorly concordant with their treatment, with average TGN levels less than 50 pmol/8×10⁸RBC over the course of the study. Among the remaining patients there was no significant difference seen between the TGN levels in those who were wild-type and those who are heterozygous for the AOX SNP.

TABLE 3 The association between each SNP and outcome. p-value for p-value for side- response effects SNP to treatment on treatment XDH 2211C > T 0.891 0.174 XDH 3030T > C 0.724 0.921 XDH 837C > T 0.933 0.046 XDH 3717G > A 0.739 0.133 XDH 2107A > G 0.391 0.631 XDH 1936A > G 1.0 0.792 MOCOS 2107C > A 0.531 0.151 MOCOS 509C > T, 0.634 0.146 1072G > A & 359G > A MOCOS 2600T > C 0.217 0.139 AOX 3404A > G 0.006 0.552 Responders are those defined as having a complete response (therapeutic target reached with no steroid therapy) and side-effects must have caused therapy to be discontinued. Statistics have been performed using the chi-squared test using the dominant model to look for clinically relevant associations. The figures given in the data cells are patient numbers presented as wild-type; heterozygous; homozygous.

D. Discussion

The association between SNPs in XDH and MOCOS and a protective effect against side-effects is interesting. No other protective SNP has ever been demonstrated in this context. This association would support the theory generated by in vitro experiments that metabolites produced by XDH can be toxic, and would suggest that the reactive oxygen species produced by XDH are responsible for a proportion of side-effects experienced on thiopurine treatment.

This provides proof of the concept that TPMT is not the only pharmacogenetically interesting enzyme in thiopurine metabolism.

With respect to the AOX SNP, this is very useful in enabling personalized selection of immunomodulators, which has long been one of the aims of pharmacogenetics in this field.

Example 2

This example illustrates the use of allopurinol and azathioprine as combination therapy.

A. Patient's Phenotype

Patients present with low levels of 6-TGN, i.e., below therapeutic levels, and moderate levels of 6-MMP when taking a normal dose of azathioprine or equivalent. Under these circumstances, the clinician will increase the dose of azathioprine, which results in a minor yet still non-therapeutic increase in 6-TGN levels, but a toxic increase of 6-MMP level. Patients have normal levels of TPMT.

B. Protocol

Patients should be genotyped for related SNPs in their xanthine dehydrogenase (XDH) gene, molybdenum cofactor sulfurase (MOCOS) gene, aldehyde oxidase (AOX) gene or a combination thereof. From these results a genotype may be used as to indicate that allopurinol should be considered as adjunct therapy.

It is to be understood that the above description is intended to be illustrative and not restrictive. Many embodiments will be apparent to those of skill in the art upon reading the above description. The scope of the invention should, therefore, be determined not with reference to the above description, but should instead be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. The disclosures of all articles and references, including patent applications, patents, PCT publications, Genbank Accession Nos., and dbSNP Accession Nos., are incorporated herein by reference for all purposes. 

1. A method for identifying a human subject who is likely protected against side-effects to a drug selected from azathioprine (AZA) or 6-mercaptopurine (6-MP), said method comprising: (a) genotyping nucleic acid in a sample from said subject for the presence of the 837 C to T mutation in the xanthine dehydrogenase (XDH) coding sequence; (b) detecting a T at position 837 in the XDH coding sequence; and (c) identifying the subject as likely to be protected against side-effects to said drug selected from azathioprine (AZA) or 6-MP when a T at position 837 in the XDH coding sequence is detected.
 2. The method of claim 1, wherein said drug is 6-mercaptopurine.
 3. The method of claim 1, wherein said drug is azathioprine.
 4. The method of claim 1, further comprising minimizing a toxicity associated with said drug.
 5. The method of claim 4, wherein said toxicity is minimized when said drug further comprises allopurinol.
 6. The method of claim 1, wherein the side-effects are selected from the group consisting of bone marrow suppression, flu-like symptoms, rash, pancreatitis, nausea and vomiting, hepatotoxicity, neutropenia, and combinations thereof.
 7. The method of claim 1, wherein said individual has a disease or disorder selected from the group consisting of an immune-mediated gastrointestinal disorder, an autoimmune disease, and graft versus host disease.
 8. The method of claim 7, wherein said immune-mediated gastrointestinal disorder is inflammatory bowel disease. 